1. Field of the Invention
This invention relates to semiconductor metrology and, more specifically, to performing measurements of critical dimensions on semiconductor surfaces using a combined stylus profilometer and atomic force microscope.
2. Description of the Relevant Art
An exemplary listing of critical dimensions (CDs) includes the width of a patterned line, the distance between two lines or devices, and the size of a contact. CDs must be monitored to maintain proper device performance. As device sizes continue to decrease at a rapid rate, it has become clear that the ability to carry out quick, inexpensive, reliable, accurate, high-resolution, non-destructive measurements of CDs in the semiconductor industry is crucial.
Measuring CDs is usually done during or after lithography. During lithography, a thin film of photoresist is applied to a semiconductor surface. This surface is then exposed to electromagnetic radiation through a mask which contains a specified pattern consisting of clear and opaque regions. Following exposure, portions of the photoresist are removed during a developing process, producing an image of the mask's pattern onto the substrate. Following development, portions of the substrate no longer covered by photoresist are etched away in order to transfer the mask's pattern onto the semiconductor substrate itself. Many factors occurring during these lithography steps affect the critical dimensions of a semiconductor device. For example, variations in the thickness of the applied photoresist, lamp intensity during the exposure process, and developer concentration all result in variations of semiconductor linewidths. Linewidth variations also occur whenever a line is in the vicinity of a step (a sudden increase in topography). Step-induced changes in linewidth are believed to be caused by at least three different factors including: a) differences in the energy transferred to the photoresist at different photoresist thicknesses, b) light scattering at the edges of the steps, and c) standing wave effects. Because these factors affect CDs greatly, it is important to monitor semiconductor devices quickly and reliably in order to guarantee acceptable performances.
Several different techniques are currently utilized to measure CDs. These include: optical microscopy, scanning electron microscopy, stylus profilometry, atomic force microscopy, and scanning tunneling microscopy. Each of the above will be discussed briefly in turn.
Optical microscopy techniques include the scanning slit technique, video-based acquisition systems, and scanning laser methods. In the scanning slit technique, white light is shined onto a sample, and a photomultiplier tube measures the reflected light passing through a narrow slit which is scanned across the surface of the wafer over a region of interest. The intensity profile output of the photomultiplier tube is used to create a densitometer profile; this profile can then be analyzed using one of several edge-sensing algorithms to generate critical dimension measurements. Video based systems operate on a similar principle. An operator chooses a region of interest on the sample which is illuminated with white light. The light reflected from the region of interest is captured by a video camera which converts an intensity profile of the region into a digital waveform. This waveform is then analyzed to determine the edges of features on the sample and to correspondingly measure critical dimensions of the sample. Scanning laser techniques operate by scanning a laser spot across a region of interest. Typically, a He--Ne laser is focused to a one-micron spot which is scanned across the features of the sample being measured. As with the optical methods discussed above, the light reflected from the sample is then measured to create a topographic profile. Methods for measuring the reflected light profile and for creating a corresponding topographic profile include: using a pair of photodiodes positioned on either side of the laser scan axis, utilizing a confocal scanned microscope and photomultiplier tube, and using laser interferometry techniques. Although resolution has been reported to be 0.25 microns with such optical techniques, several problems limit their capabilities. The largest problem with these optical techniques is their accuracy limitations. Factors contributing to limited accuracy include: limited optical resolution of lenses, non-uniform illumination, problems with coherence, poor mechanical stability, and optical aberrations. Problems associated with the above methods' detection techniques include poor spatial resolution, slow response time, and stability or alignment problems. Although scanning electron microscopy achieves greater accuracy than the above optical methods, it too suffers from some of the same problems as well as introducing new problems in performing CD measurements.
A scanning electron microscope (SEM) operates by creating a beam of electrons which are accelerated to energies ranging from several hundred to several thousand electron volts. The electron beam is focused to a small diameter and scanned across a region of interest of a sample. As the beam strikes the sample's surface, low energy secondary electrons are emitted. The yield of secondary electrons depends on many factors such as the work function of the material, the topography of the sample, and the curvature of the surface. Because different materials may have significantly different work functions, the yield of secondary electrons may be used as a contrast mechanism to distinguish between different materials on a surface. Because changes in topography affect secondary electron yield, one may similarly measure a change in height along the sample's surface. This ability to detect rapid changes in topography is crucial for making CD measurements, for one must, for example, first determine the location of trench edges before measuring the dimensions of the trench. Electron current resulting from the surface-emitted secondary electrons is detected and used to correspondingly control the intensity of pixels on a monitor attached to the SEM. An image of the region being studied is formed by synchronously scanning the electron beam and the screen of the monitor. Even though SEMs can achieve resolution in the range of angstroms, several disadvantages hinder their ability to become an optimal tool for measuring CDs.
SEM weaknesses include the necessity of coating the sample with a conductive layer. A coating must be applied in order to avoid the affects of surface charging which occurs when a high accelerating voltage is used in the SEM. In regards to measuring CDs, the presence of a coating may change the profile of the sample being studied, negatively impacting the measurement obtained. Having to place the sample in a vacuum environment is another drawback to SEM techniques. Vacuum equipment is expensive and may require frequent maintenance; in addition, the vacuum chamber usually limits the size and orientation of the sample which may fit into the SEM. To perform vertical profile CD measurements with an SEM, one must first cleave a sample along the region of interest in order to perform SEM measurements. This requirement of cleaving is greatly limiting, for it means that the measurement technique is time consuming, destructive, and requires the acquired skill of a technician or the presence of an expensive automatic cleaving tool. SEM measurements are also limited in spatial resolution due to e-beam interactions which depend on the materials being measured. For at least these reasons, SEM is currently not an optimal method for CD measurements.
Stylus profilometer techniques are one of the most common methods for measuring CDs. A stylus profilometer is a probe which is drawn across the sample--as the stylus (usually employing a diamond tip) encounters a change in topography, a signal variation based on differential capacitance or inductance is sensed and yields an indication of the step height. Such profilometer techniques are well known in the art and are used in several commercial devices, such as the Sloan Dektak and Tencor Alpha-step machines. Advantages of using a stylus profilometer include ease of use, price, and speed. A major disadvantage, however, is due to the relatively large size of the probe itself. As the stylus is drawn across the surface of the sample, a sudden change in topography due to, for example, a trench cannot be tracked due to the size of the tip and the relatively high scanning speed.
The atomic force microscope (AFM) is a combination of the principles from scanning tunneling microscopy (to be discussed briefly below) and stylus profilometry. In atomic force microscopy, a sharp tip attached to a cantilever is scanned across the surface of a sample. The deflections of the cantilever/tip system are used to measure the small force created by the tip's close proximity to the sample. By using a feedback mechanism to control the separation distance between tip and sample, this small force (&lt;10.sup.-15 Newtons) may be kept constant while scanning across the surface. With a constant force, the tip motion in the z direction will mimic the topography of the sample. Thus, while scanning across the sample, the signal required to maintain a constant cantilever deflection can be recorded, and this signal corresponds to the topography of the scanned surface.
Many different techniques are used for the feedback mechanism in atomic force microscopy. The purpose of each of these differing methods is to accurately measure the cantilever's deflection. With the deflection measured accurately, the feedback mechanism can accurately control the sample/tip distance such that the cantilever deflection remains constant. Early atomic force microscopes used scanning tunneling microscopes to measure the deflection of the cantilever. Since then, there have been many other methods used to detect the deflection of the cantilever/tip system. Usually, optical means are used to measure the cantilever's deflection. For example, a laser may be shined onto the back side of a cantilever and onto an interferometer. The reflected light from the cantilever may interfere with an unreflected beam and produce interference fringes which may be used to determine the deflection of the cantilever. Another way to detect the motions of the cantilever is to shine laser light onto the back of a cantilever and collect the reflected light with a position sensitive detector. Such means are well known in the art--many atomic force manufacturers build equipment employing such means.
Because of their central role, it is important to discuss piezoelectric scanners in regard to atomic force microscopy. Scanners both move along the plane of the sample and allow the tip to track a sample's topography. Most scanners used today are produced from a piezoelectric material made of a lead zirconium titanate (PZT) tube that has been polarized radially. The tube is quartered parallel to its axis and electrodes are placed at each quarter. When a bias is applied between opposite quarters, the tube will extend or contract. When a bias is applied to only one quarter the tube will bend. Therefore, a single tube scanner can be used to both scan the surface and also to control the z-height of the tip, depending on how the applied biases are arranged. An inherent problem with atomic force microscopy scanners (and piezoelectric scanners in general) is their nonlinearity. Although an atomic force microscope's scanner would ideally respond linearly with the applied voltage, scanners exhibit intrinsic non-linearity. A quantitative way to measure the extent of intrinsic nonlinearity is to measure the ratio of the maximum deviation from linearity to the liner extension. In commercial atomic force microscopes, intrinsic nonlinearity can range from two to ten percent. Because scanners do not respond linearly, the features of a sample may appear to have different spacing. Out of the plane of the sample, intrinsic nonlinearity causes slight (2-10%) discrepancies in topographic height measurements. Besides non linearity, atomic force microscopy scanners suffer from problems due to hysterisis, creep, and cross coupling. Hysteresis refers to the following phenomenon: if the voltage applied to a piezo is increased to a certain value and then brought back down to the original voltage, the piezo will not retract along the same path which it extended. In commercial instruments, hysteresis can be as high as 20%. Creep refers to the problem encountered by a scanner when reacting to a swift change in topography. Instead of responding instantly to a sudden change in voltage (brought about by a sudden change in topography), the piezo material responds in two steps. The first of these steps usually occurs in less than a millisecond while the second step can take as long as 100 seconds in some instruments. The effects of creep can be considered by thinking of scanning a symmetric step. As the piezo extends or contracts along the sidewalls of the step, creeping occurs; to maintain the tracking of the step, the feedback system applies a voltage countering the creep. This counter voltage appears as a ridge or shadow of the step. Cross-coupling refers to the fact that as a piezo is scanned across the x-y plane, there exists a z-component of the piezo extension. Thus, the scanner can thought to be moving in an arc. Although these factors may present some problems for making CD measurements, they are diminished by the excellent resolution and accuracy capabilities of an atomic force microscope.
Larger drawbacks to using atomic force microscopes to measure CDs relate to the microscopes' tips. In order to measure CDs with very high resolution, tips sharpened with a focused ion beam are often employed. These high aspect ratio tips allow for accurate measurements of surfaces having rapid changes in topography--such as samples being measured for critical dimensions. Although having their advantages, these tips are extremely fragile and must be replaced often if large scans are performed. Thus, atomic force microscopes present problems relating to reliability and maintenance. In addition, using atomic force microscopes to measure CDs is a very slow process. Although high resolution, the scans can be very slow, reducing throughput. Such reductions in throughput are always sought to be avoided.
As mentioned earlier, scanning tunneling microscopes (STMs) share many qualities with atomic force microscopy. Like AFMs, STMs track the surface of a sample with a sharp tip controlled by piezoelectric scanners. However, STMs, instead of a force gradient, use tunneling current between the tip and the sample as a feedback mechanism. The sharp, metallic tip (ideally the tip is a single atom at the end of a thin wire such as an etched tungsten wire) is brought sufficiently near a sample until a tunneling current is established between the tip and sample. As the tip is scanned across the surface, one may maintain a constant current via a feedback loop (constant current mode) while measuring the z response of the piezo, or the z position of tip may remain constant while the tunneling current is monitored (constant-height mode). Either way, the feedback loop (current or z response of the piezo) is correlated with the topography of the sample. Like AFMs, STMs suffer the same problems inherent with any piezoelectric scanners while scanning over features exhibiting large changes in vertical height. STMs also are not well-suited for CD measurements because the sample being studied must be conductive in order to establish a tunneling current between the tip and sample. Thus, one may not use an STM to scan over the insulators so common in the semiconductor industry. Also, most commercial STMs require a high-vacuum environment in order to achieve adequate resolution. Such vacuum equipment requires much maintenance and can be quite expensive.
It would thus be advantageous have a method and apparatus which would allow for quick, reliable, inexpensive, non-destructive, high-resolution CDs measurements.